A study conducted by a US military adviser has found that drone strikes in Afghanistan during a year of the protracted conflict caused 10 times more civilian casualties than strikes by manned fighter aircraft.

The new study, referred to in an official US military journal, contradicts claims by US officials that the robotic planes are more precise than their manned counterparts.

It appears to undermine the claim made by President Obama in a May speech that "conventional airpower or missiles are far less precise than drones, and likely to cause more civilian casualties and local outrage".

Drone strikes in Afghanistan, the study found, according to its unclassified executive summary, were "an order of magnitude more likely to result in civilian casualties per engagement."

Larry Lewis, a principal research scientist at the Center for Naval Analyses, a research group with close ties to the US military, studied air strikes in Afghanistan from mid-2010 to mid-2011, using classified military data on the strikes and the civilian casualties they caused. Lewis told the Guardian he found that the missile strikes conducted by remotely piloted aircraft, commonly known as drones, were 10 times more deadly to Afghan civilians than those performed by fighter jets.

Lewis, an adviser to the military's Joint Staff, conducted six previous studies of civilian casualties and other episodes in Afghanistan for the military.

"The fact that I had been looking at air operations in Afghanistan for a number of years led me to suspect that what I found was in fact the case," Lewis said.

But "the potential for [citizens to be] surprised" by the higher rates of civilians killed by drones led Lewis and his co-author, Sarah Holewinski of the non-governmental organization Center for Civilians in Conflict, to refer to Lewis' findings in an article for Prism, a journal published by the Center for Complex Operations at the Defense Department's National Defense University.

Lewis said he could not provide specific figures about the numbers of civilian casualties caused by drones and manned aircraft in Afghanistan, citing classified information. Nor does the Prism article specifically refer to the finding that drones are 10 times likelier to kill civilians than manned aircraft are.

"These findings show us that it's not about the technology, it's about how the technology is used," said Holewinski. "Drones aren't magically better at avoiding civilians than fighter jets. When pilots flying jets were given clear directives and training on civilian protection, they were able to lower civilian casualty rates."

Yet the demand for additional drone strikes by commanders in the war zone creates pressure to reduce training, Holewinski and Lewis note.

"Adding or improving training on civilian casualty prevention is a resource decision in direct tension with the increasing demand for more UAS [unmanned aerial systems] and more operations, since additional training on civilian protection means time must be taken from somewhere else including the mission itself," Lewis and Holewinski write in their Prism article.

While the drone strikes remain classified, several senior Obama administration officials and their congressional allies have described them as notable for their precision. John Brennan, now the CIA director responsible for the agency's drones, said in 2012 they provide "targeted strikes against specific al-Qaida terrorists". While defending the strikes as legal and "targeted", Obama conceded in May that "US strikes have resulted in civilian casualties, a risk that exists in all wars". Dianne Feinstein, the California Democrat who chairs the Senate intelligence committee, said in February that drones kill only "single digits" worth of civilians annually.

"This data from Afghanistan, if accurate, suggests that the precision may be overstated in some contexts, and requires us to dig deeper into strike practices," said Sarah Knuckey, an adviser to the United Nations special rapporteur on extrajudicial executions, who is currently investigating the impact of drone strikes on civilians.

"The key question raised is: What explains the discrepancy between civilian casualties from UAV [unmanned aerial vehicles] and manned aircraft strikes? To enable fair external assessment, the government should release the underlying data, redacted as necessary."

The period Lewis researched was the most intense era for air strikes during the 12-year-long Afghanistan war. General David Petraeus, commander from mid-2010 to mid-2011, reversed restrictions on air strikes imposed by his predecessor, Stanley McChrystal, designed to tamp down Afghan outrage about dead civilians. By February 2011, air force pilots, both in and outside the cockpit, launched 10 bombing missions daily, nearly double the rate of the previous year.

The air war in Afghanistan has declined significantly since Petraeus' departure and the end of the troop surge he implemented. But Afghanistan still remains the central battleground for US drone strikes. As of 6 December 2012, the US launched 447 drone strikes in Afghanistan that year, up 5% from 2011. By contrast, there were 48 drone strikes in 2012 in Pakistan, according to a tally kept by the New America Foundation think tank.

"Under the laws of war, if there are two weapon systems that offer roughly equal capacity to overcome an adversary, the weapon which could be expected to inflict the least civilian casualties must be employed," said Ryan Goodman, a professor at New York University school of law. "This is a widely understood rule in the laws of war."

Lewis and Holewinski were reluctant to draw conclusions about the rates of civilian casualties caused by drones outside of Afghanistan.

But Holewinski noted that it is far easier for US forces in Afghanistan to conduct post-strike investigations that determine whether it killed militants or civilians than it is in Pakistan, Yemen or anywhere else the CIA and the military conducts drone strikes.

In those areas, "the only information you're really getting is from the drone," Holewinski said. "You're looking from 10,000 feet or wherever the drone is, and counting the bodies or the cars destroyed. How do you know who was in them, [and] whether they were civilians or combatants?"

Lewis said the "general principle" about the relative imprecision of drones outside of Afghanistan was more important than the specific ratio of deaths they caused compared to manned fighters.

"I've never seen any use of any weapons system in any kind of warfare that doesn't have room for improvement," Lewis said.